1. Field of the Invention
This invention relates to a process for producing a high impact styrene resin by continuous bulk polymerization, and to a high impact styrene resin obtained by this process. More specifically, it relates to a process for producing by continuous bulk polymerization a high impact styrene resin having excellent strength in which a grafted rubbery polymer in the resin has a narrow particle size distribution, and to a high impact styrene resin obtained by this process.
2. Description of the Prior Art
In the production of a high impact styrene resin by graft polymerizing a styrene monomer in the presence of a rubbery polymer, how to control the dispersion of the rubbery polymer uniformly as fine particles and the stabilization of the rubbery polymer particles during the graft polymerization is especially important in determining the quality of the resulting resin. The particle size and particle size distribution of the grafted rubbery polymer particles dispersed in the final resin, and the grafting ratio of styrene greatly affect the impact strength and gloss of the final resin.
For the industrial production of high impact styrene resin, a batchwise bulk-suspension polymerization method and a continuous bulk polymerization method have generally been employed. The continuous method, however, has been found to be superior in productivity and economy, and has gained widespread acceptance.
Usually, in the continuous bulk polymerization method, it is general practice to provide a plurality of stirred tank reactors and to perform polymerization while continuously feeding a starting material solution.
More specifically, there was proposed a method in which in an initial polymerization step, the rubbery polymer is divided into fine particles and dispersed by dynamic mixing in a stirred tank reactor with a stirrer, and thereafter, the polymer solution is continuously transferred to a plurality of stirred tank reactors, and the polymerization is allowed to proceed with stirring.
It has been pointed out however that the continuous bulk polymerization using the tank reactors has the defects attributed to the dynamic mixing by the stirrer. Specifically, when the viscosity of the polymer solution in the tank rises as the polymerization proceeds, the power or the strength of the stirring vanes has to be increased. To avoid it, the polymerization conversion of the styrene monomer in the tank must be inhibited as the polymerization proceeds. Alternatively, if excessive stirring and mixing is carried out an excessive shearing force is exerted on the rubbery polymer dispersed as fine particles. Consequently, the dispersed fine particles of the rubbery polymer are broken, and the particle size distribution of the rubbery polymer particle becomes broad. As a result, the strength of the final product decreases. Another defect is that since the stirred tank reactor has an excessive capacity because of its structure, a long period of time is required, for example, in switching from one brand to another in the production of resins in which the rubbery polymer has different particle diameters. In an attempt to remove these defects of the stirred tank reactor, there have been made some suggestions including the structure of stirring vanes, the performance of pre-polymerization, or the combination of it with a tubular reactor or a tower-type reactor. But these suggestions have not proved to be sufficient for removing the above defects.
Some proposals, for example the improvement of stirring blades, the use of a tubular reactor having a stirrer and the use of a recycle line, were made in an attempt to remove these defects of the stirred tank reactor, but no entirely satisfactory improvement has yet been achieved.
For example, in U.S. Pat. No. 3,660,535, the dispersion of the rubbery polymer and the formation of fine particles are controlled by stirring and recycling the initial-stage polymer solution by using a vertical tubular reactor (towered reactor) having a stirrer and a recycle line. If, however, the amount of the initial-stage polymer solution recycled is increased in this method in an attempt to disperse the rubbery polymer uniformly, the rubbery polymer tends to have an increased average particle size and a broadened particle size distribution. Hence, to obtain a resin having a small average particle size and a narrow particle size distribution, it is necessary to decrease the amount of the initial-stage polymer solution recycled and consequently, the dispersion of the rubbery polymer is not effected well. Thus, it is difficult in this method to produce a resin having a small average particle size and a narrow particle size distribution with uniformly dispersed rubbery polymer particles, namely a resin having excellent gloss and impact strength. Furthermore, it may be possible in this method to increase the rotating speed of the stirrer as means for uniformly dispersing the rubbery polymer and decreasing its average particle size. This, however, results in an excessive shearing force on the rubbery polymer, a broadening of its particle size distribution and further in a reduction in its impact strength, and a resin having excellent gloss and impact strength is difficult to produce.
In view of this state of the art, the present inventors studied a method of continuously producing high-impact polystyrene in tubular reactors having fixedly set therein a plurality of mixing elements having no moving parts.
On the utilization of a tubular reactor in a continuous polymerization, U.S. Pat. No. 4,275,177 proposes a process which uses a tank reactor having a stirrer for carrying out an initial-stage polymerization and a tubular reactor having fixedly set therein a plurality of mixing elements having no moving parts for carrying out the main polymerization. In this process, however, the defects attributed to the dynamic mixing by the stirrer within the tank cannot be removed, and an excessively high shear owing to the intense stirring and mixing is exerted to broaden the particle size distribution of the rubbery polymer particles. The grafting ratio of styrene is not increased, and the results are still unsatisfactory.